1. Field of the Invention
The present invention relates to a method for producing hydrophobic silica fine powder which can be used as a thickener for coatings, adhesives and synthetic resins, as a reinforcement for plastics, and to improve flowability in toners for copiers.
2. Prior Art
West German Patent No. 1163784 discloses that pyrogenic silica (silicon dioxide) having silanol groups on the surface is hydrophobized with dimethyldichlorosilane as a hydrophobizing agent in a parallel flow fluidized bed at 400 to 600xc2x0 C. in the presence of steam while feeding nitrogen for fluidization. This process is uneconomical since a substantial amount of unreacted silane hydrophobizing agent is carried over with the waste gases. West German Patent P3211431.1 describes that waste gases resulting from hydrophobizing reaction are fed back to the hydrophobizing process whereby unreacted silane is also fed back to the hydrophobizing process, which enables to reduce the amount of silane hydrophobizing agent used. This method, however, is complex and requires to control the flow rate and pressure of waste gases. JP-A 6-206720 (U.S. Pat. No. 5,372,795) discloses that waste gases containing unreacted silane are recycled to a combustion chamber where they are burned. This method also requires to control the flow rate of waste gases since combustion fluctuates unless the amounts of air/hydrogen/starting silane and waste gases are controlled as appropriate.
In a process of hydrophobizing silica fine powder in a fluidization vessel, the silica is often treated with an organohalosilane (referred to simply as xe2x80x9csilane,xe2x80x9d hereinafter), typically dimethyldichlorosilane as hydrophobizing agent. The resulting waste gases contain a noticeable amount of unreacted silane hydrophobizing agent. The presence of silane in the waste gases leads to a number of practical obstacles when the silane-containing waste gases are treated with a scrubber, such as the formation of foam, which cannot be easily removed with filters.
An object of the invention is to provide a method for producing hydrophobic silica fine powder at an improved reaction efficiency of hydrophobizing agent and a reduced burden on waste gas treatment.
The invention concerns to a method for producing hydrophobic silica fine powder by pyrolyzing a silane compound to form a silica fine powder and hydrophobizing the silica fine powder with an organohalosilane in a fluidization vessel. In studies to reduce the amount of unreacted silane in waste gases without complicating the process and its control, we have found that by feeding a portion of silica resulting from flame hydrolysis to a waste gas line from the hydrophobizing step, contacting the silica with unreacted silane being carried over with the waste gases, and feeding the silica to the hydrophobizing step, the amount of unreacted silane in waste gases can be reduced, thereby alleviating the burden on waste gas treatment; and unreacted silane is fed back to the fluidization vessel along with the silica, thereby increasing the reaction efficiency of silane.
In an experiment, a cyclone and a bag filter were installed in the waste gas line to recover silica that had flown out from the fluidization vessel. On measuring the temperature at various places in the waste gas line, we have found that, if the temperature of the waste gas line is maintained at 100xc2x0 C. or higher, the moisture present in the waste gases does not condense and undesirable products such as gels or oils due to moisture and unreacted silane do not form. In particular, the absence of gel or oil formation on the filter fabric in a bag filter keeps the filter fabric free of clogging, making it possible to carry out continuous operation. By maintaining the waste gases at 100xc2x0 C. or higher, essentially 100% of fugitive silica in the waste gases can be recovered with the cyclone and bag filter. It has been found that the amount of unreacted silane in the waste gases can be reduced by feeding a portion of silica fine powder resulting from pyrolysis to the waste gas line, contacting the silica with unreacted silane, collecting the silica in the cyclone and bag filter, and feeding the collected silica to the fluidization vessel where it is hydrophobized. This is presumably because the unreacted silane is brought in effective contact with the silica upon collection of the silica with the cyclone and bag filter fabric.
Accordingly, the invention provides a method for producing hydrophobic silica fine powder. A silane compound is pyrolyzed to form a silica fine powder. The silica fine powder is then hydrophobized with an organohalosilane in a fluidization vessel. A portion, specifically 3 to 20% by weight of the silica fine powder is directly fed to a waste gas line from the fluidization vessel and collected with a cyclone and bag filter. The collected powder is fed to the fluidization vessel where it is hydrophobized. The cyclone and the bag filter are preferably held at a temperature of at least 100xc2x0 C.
In the process involving flame hydrolysis of silane to form pyrogenic silica (silicon dioxide fine powder) and hydrophobizing the pyrogenic silica in a fluidization vessel with a hydrophobizing agent, the invention is to feed a portion of the pyrogenic silica to the waste gas line where the silica is contacted with the unreacted hydrophobizing agent carried over with the waste gases and then feed the silica to the fluidization vessel. As a result, the amount of unreacted hydrophobizing agent in the waste gases is reduced, whereby the burden on waste gas treatment is alleviated. Additionally, the unreacted hydrophobizing agent is fed back to the fluidization vessel along with the silica, whereby the reaction efficiency of hydrophobizing agent is increased.